Magnetic tape drives are commonly used to provide significant data storage capacity and serve as an inexpensive alternative to disk drives whenever sequential data access is acceptable. Data is recorded inside a layer of ferromagnetic material deposited on a long strip of tape which is wound between two hubs and enclosed within a cartridge of appropriate dimensions. To perform data transfer to and from the tape, the cartridge is inserted into a tape drive bringing the tape into intimate contact with a read/write head. Data is recorded or accessed sequentially, according to a predetermined format, as the tape is advanced past the read/write head by means of a pair of motors, each coupled to one of the hubs, which act to maintain a desired tape speed. In addition, motion of the trailing motor is determined relative to the motion of the advance motor to provide the necessary tape tension for close contact between the tape and the head in order to obtain good data signal quality.
The need to maintain highly reliable data transfer requires good signal to noise ratio and, consequently, large amplitudes of the readback signal which is effected through a direct physical contact between the tape and the read/write head throughout the operation of the tape drive. The contact is obtained by tensioning the tape to conform to the contour of the read/write head. Thus tensioning is effected through appropriate simultaneous control of the respective speeds of the advance motor and the trailing motor. Weak tape tension can lead to unreliable contact and intermittent tape separation from the read/write head which reveals itself in the form of a low amplitude readback signal and poor data transfer reliability. Significantly overtensioning the tape, however, can lead to media loss due to plastic deformation of the tape base material. More commonly, slight excesses in tape tension over time lead to rapid read/write head wear, shortening the useful life of the tape drive.
In the past, tape tension has been controlled by such means as: feedback loops using tension sensors, vacuum columns, and/or mechanical tensioners. These methods have attempted to keep tape tension constant. However, these approaches do not account for variations in physical characteristics such as: tape dimensions and properties, mechanical variations in the tape drive mechanism, and contour of the read/write head. Since direct tape to head contact produces frictional wear of the head material, appreciable changes in the head geometry occur over time. As a result, tape tension values which were optimal at the outset no longer provide adequate conformance of the tape to the changing head contour with consequent loss in the data signal quality and tape drive reliability due to higher data error rates.
An additional difficulty experienced with tape drive tension determination occurred whenever a variety of tapes were used on the same drive over a range of operating conditions. Elastic properties of the tape can be expected to change over time, dependent upon environmental effects such as temperature and ambient humidity as well as with extended use. Moreover, tape properties, such as tape thickness, elasticity and magnetic characteristics, vary across production lots and from one tape product to another. With the previous methods of tape tension control, the tape drive operation over a range of ambient conditions, e.g. temperature, humidity, variation of motor constants, and tape characteristics, resulted in less reliable data transfer, limiting the scope of potential drive applications.
It is desired to have a method of adjusting the tape tension to adapt to variations in the tape drive physical characteristics as well as environmental conditions.